Progressive scan television receivers have been proposed wherein the horizontal line rate is doubled and each line of video is displayed twice thereby providing a displayed image having twice the usual number of scan lines and thus reduced visibility of vertical line structure. In one known form of progressive scan receiver each incoming horizontal line of video signal is stored in one of two line memorys. As one line is being stored in one memory the line previously stored in the other memory is recovered or "read" twice thereby providing two lines of time compressed video within one standard line interval. The memory output is applied to a display having a doubled horizontal sweep rate synchronized with readout of the memory thereby doubling the number of displayed lines of the video signal.
An example of the progressive scan receiver in which additional scan lines for the display are obtained by interpolation from the original scan lines is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled "TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS" which issued Aug. 23, 1983. A receiver in which the added lines are replicas of the original lines is described by R. A. Dischert in U.S. Pat. No. 4,415,931 entitled "TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES" which issued Nov. 15, 1983.
It is recognized in the aforementioned Powers patent, for example, that field or frame memorys may be used to advantage in doubling the line rate in a progressive scan receiver. In such systems an entire frame of the video input signal is stored in a memory and recovered during one field interval for display thereby providing a full 525 lines of the incoming video signal during each field of the displayed signal. The advantage of such an arrangement is that the displayed signal is not subject to interpolation errors and preserves the full resolution of the transmitted signal. A disadvantage of such arrangements is that visual artifacts tend to be produced such as double images and serrated edge effects when there is motion in the scene due to field-to-field differences of the information stored in the memory.
A solution to the problem of motion artifacts proposed by Powers is to apply the video input signal to a motion detector and utilize the output of the motion detector to automatically switch between frame store progressive scan processing and line store progressive scan processing when motion is present. Motion is detected in the Powers arrangement by comparing a number of currently received picture elements (pixels) with corresponding picture elements delayed by one field and summing the result of the comparisons to produce a weighed average. The average is compared against a minimum motion threshold value to generate the motion indicating output signal for selecting between line and frame type progressive scan processing.